Effect of blend composition on ternary blend organic solar cells using a low band gap polymer

This work investigates the influence of blend composition in ternary blend bulk heterojunction organic solar cells composed of poly(3-hexylthiophene-2,5-diyl) (P3HT), poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b:3,4-b′]dithiophene-siloe 2,6-diyl]] (Si-PCPDTBT) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). The use of the low band gap Si-PCPDTBT vastly improves the spectral response, when compared to a P3HT:PC71BM binary system. The optimum blend composition occurred at a total polymer concentration of 50 wt%. At low polymer concentrations, the P3HT phase was amorphous in nature. Increasing the polymer content led to the formation of crystalline polymer domains, as evidenced by XRD measurements. This significantly enhanced the charge carrier transport throughout the active layer. XPS depth profiles indicated that variations in the polymer content also influenced the mixing between the Si-PCPDTBT and the P3HT host matrix. This analysis showed that the 50 wt% was conducive to a larger interaction between the two polymers. A comprehensive analysis of the relative contributions of each molecule to the photoluminescence suggested that the polymer concentration not only affects the film microstructure, it also influences the photoluminescence quantum yield of the blend. This is caused by alterations to the recombination mechanisms occurring in the constituent materials, which, in turn, influences photocurrent generation. This result shows that the overall polymer content must be chosen carefully to strike a delicate balance between improved absorption and effective charge generation and collection.